Mixing light from light emitters that emit light of different colors (e.g., solid state light emitters, such as light emitting diodes and luminescent materials) may allow for the production of light of a desired hue, e.g., white light of a desired color temperature, and can in some cases provide good CRI Ra and/or high energy efficiency. For instance, in some cases, non-saturated bluish yellow light (e.g., from a light source that comprises a light emitting diode that emits saturated blue light and a luminescent material that emits non-saturated yellowish-green light) and red light (e.g., from a light source that comprises a light emitting diode that emits saturated red light) can be mixed to provide very high CRI Ra white light (CRI Ra of 90 or greater) at very high efficiencies. The term “hue” is used herein to refer to the color of light (e.g., light emitted by a light emitter or light that is a mixture of light emitted from two or more light emitters) corresponding to a particular color point on a Chromaticity Diagram (discussed below).
One difficulty with mixing light of different colors can occur if the respective light sources (that emit light of different colors) respond differently to variations in operating parameters. For example, if the respective light sources that emit light of different colors are from different materials systems, such as InGaN and AlInGaP, the output characteristics may respond differently to changes in operating temperature or current, or their output may change with time at different rates. Unless these changes are taken into account in the drive circuitry of the light sources (or in some other way), which can be challenging and/or expensive, the color point of the output light can shift with changes in operating parameters.
There exist lighting devices that comprise one or more light emitters that emit bluish-yellow light (the “bluish-yellow light emitters”) and one or more light emitters that emit red light (the “red light emitters” or the “red light emitting diodes”), in which the light output from the respective light emitters is mixed to produce output light (intended to be white light). These devices include a sensor that is responsive to only the bluish-yellow light. These devices further include a thermistor circuit to compensate the current supplied to the light emitters that emit red light based on the roll-off of light output with increasing temperature that occurs with the red light emitting diodes. In these devices, the current supplied to the red light emitting diodes is adjusted based on the brightness of the bluish yellow-light detected by the sensor, and based on a prediction of the performance of the red light emitting diodes (based on the temperature detected by the thermistor), in an attempt to maintain the color point of the light emitted by the lighting device. (see, e.g., LR6, LR24, LRP38 products available from Cree, Inc. (and described at www.cree.com/products.aspx; also, see:    U.S. patent application Ser. No. 12/117,280, filed May 8, 2008 (now U.S. Patent Publication No. 2008/0309255), the entirety of which is hereby incorporated by reference as if set forth in its entirety;    U.S. patent application Ser. No. 12/469,819, filed on May 21, 2009 (now U.S. Patent Publication No. 2010-0102199), the entirety of which is hereby incorporated by reference as if set forth in its entirety;    U.S. patent application Ser. No. 11/755,149, filed May 30, 2007 (now U.S. Patent Publication No. 2007/0278974), the entirety of which is hereby incorporated by reference as if set forth in its entirety;    U.S. Patent Application No. 61/245,688, filed on Sep. 25, 2009, the entirety of which is hereby incorporated by reference as if set forth in its entirety;    International Patent Application No. PCT/US10/49564, (now PCT Publication No. WO/2011/037877), filed Sep. 21, 2010, the entirety of which is hereby incorporated by reference as if set forth in its entirety;    U.S. patent application Ser. No. 12/541,215, filed on Aug. 14, 2009 (now U.S. patent Publication No. 2011-0037409), the entirety of which is hereby incorporated by reference as if set forth in its entirety;    U.S. patent application Ser. No. 12/469,828, filed on May 21, 2009 (now U.S. Patent Publication No. 2010-0103678), the entirety of which is hereby incorporated by reference as if set forth in its entirety; and    U.S. patent application Ser. No. 12/475,850, filed on Jun. 1, 2009 (now U.S. Patent Publication No. 2009-0296384), the entirety of which is hereby incorporated by reference as if set forth in its entirety.The LR6, LR24 and LRP38 products are tuned to a specific color point at the factory.
In many cases, the bluish-yellow light emitters in the LR6, LR24 and/or LRP38 products degrade (i.e., decrease in their brightness, all other conditions being equal) more rapidly than the red light emitting diodes. If in a specific lighting device the red light emitting diodes degrade more rapidly than the bluish-yellow light emitters, the mapping of the current to output of the red light emitting diodes can be incorrect and/or can change over time, as a result of which the circuit that is included to maintain the color point of the output light may actually end up altering the color point.
There is an ongoing effort to develop systems that are more energy-efficient. A large proportion (some estimates are as high as twenty-five percent) of the electricity generated in the United States each year goes to lighting, a large portion of which is general illumination (e.g., downlights, flood lights, spotlights and other general residential or commercial illumination products). Accordingly, there is an ongoing need to provide lighting that is more energy-efficient.
Solid state light emitters (e.g., light emitting diodes) are receiving much attention due to their energy efficiency. It is well known that incandescent light bulbs are very energy-inefficient light sources—about ninety percent of the electricity they consume is released as heat rather than light. Fluorescent light bulbs are more efficient than incandescent light bulbs (by a factor of about 10) but are still less efficient than solid state light emitters, such as light emitting diodes.
In addition, as compared to the normal lifetimes of solid state light emitters, e.g., light emitting diodes, incandescent light bulbs have relatively short lifetimes, i.e., typically about 750-1000 hours. In comparison, light emitting diodes, for example, have typical lifetimes between 50,000 and 70,000 hours. Fluorescent bulbs have longer lifetimes than incandescent lights (e.g., fluorescent bulbs typically have lifetimes of 10,000-20,000 hours), but provide less favorable color reproduction. The typical lifetime of conventional fixtures is about 20 years, corresponding to a light-producing device usage of at least about 44,000 hours (based on usage of 6 hours per day for 20 years). Where the light-producing device lifetime of the light emitter is less than the lifetime of the fixture, the need for periodic change-outs is presented. The impact of the need to replace light emitters is particularly pronounced where access is difficult (e.g., vaulted ceilings, bridges, high buildings, highway tunnels) and/or where change-out costs are extremely high.
LED lighting systems can offer a long operational lifetime relative to conventional incandescent and fluorescent bulbs. LED lighting system lifetime is typically measured by an “L70 lifetime”, i.e., a number of operational hours in which the light output of the LED lighting system does not degrade by more than 30%. Typically, an L70 lifetime of at least 25,000 hours is desirable, and has become a standard design goal. As used herein, L70 lifetime is defined by Illuminating Engineering Society Standard LM-80-08, entitled “IES Approved Method for Measuring Lumen Maintenance of LED Light Sources”, Sep. 22, 2008, ISBN No. 978-0-87995-227-3, also referred to herein as “LM-80”, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein.
LEDs also may be energy efficient, so as to satisfy ENERGY STAR® program requirements. ENERGY STAR program requirements for LEDs are defined in “ENERGY STAR® Program Requirements for Solid State Lighting Luminaires, Eligibility Criteria—Version 1.1”, Final: Dec. 19, 2008, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein.
General illumination devices are typically rated in terms of their color reproduction. Color reproduction is typically measured using the Color Rendering Index (CRI Ra). CRI Ra is a modified average of the relative measurements of how the color rendition of an illumination system compares to that of a reference radiator when illuminating eight reference colors, i.e., it is a relative measure of the shift in surface color of an object when lit by a particular lamp. The CRI Ra equals 100 if the color coordinates of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the reference radiator.
Daylight has a high CRI (Ra of approximately 100), with incandescent bulbs also being relatively close (Ra greater than 95), and fluorescent lighting being less accurate (typical Ra of 70-80). Certain types of specialized lighting have very low CM (e.g., mercury vapor or sodium lamps have Ra as low as about 40 or even lower). Sodium lights are used, e.g., to light highways—driver response time, however, significantly decreases with lower CRI Ra values (for any given brightness, legibility decreases with lower CRI Ra).
In order to encourage development and deployment of highly energy efficient solid state lighting (SSL) products to replace several of the most common lighting products currently used in the United States, including 60-Watt A19 incandescent and PAR 38 halogen incandescent lamps, the Bright Tomorrow Lighting Competition (L Prize™) has been authorized in the Energy Independence and Security Act of 2007 (EISA). The L Prize is described in “Bright Tomorrow Lighting Competition (L Prize™)”, May 28, 2008, Document No. 08NT006643, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein. The L Prize winner must conform to many product requirements including light output, wattage, color rendering index, correlated color temperature, expected lifetime, dimensions and base type.
The color of visible light output by a light emitter, and/or the color of blended visible light output by a plurality of light emitters can be represented on either the 1931 CIE (Commission International de I'Eclairage) Chromaticity Diagram or the 1976 CIE Chromaticity Diagram. Persons of skill in the art are familiar with these diagrams, and these diagrams are readily available (e.g., by searching “CIE Chromaticity Diagram” on the internet).
The CIE Chromaticity Diagrams map out the human color perception in terms of two CIE parameters x and y (in the case of the 1931 diagram) or u′ and v′ (in the case of the 1976 diagram). Each point (i.e., each “color point”) on the respective Diagrams corresponds to a particular hue. For a technical description of CIE chromaticity diagrams, see, for example, “Encyclopedia of Physical Science and Technology”, vol. 7, 230-231 (Robert A Meyers ed., 1987). The spectral colors are distributed around the boundary of the outlined space, which includes all of the hues perceived by the human eye. The boundary represents maximum saturation for the spectral colors.
The 1931 CIE Chromaticity Diagram can be used to define colors as weighted sums of different hues. The 1976 CIE Chromaticity Diagram is similar to the 1931 Diagram, except that similar distances on the 1976 Diagram represent similar perceived differences in color.
The expression “hue”, as used herein, means light that has a color shade and saturation that correspond to a specific point on a CIE Chromaticity Diagram, i.e., a point that can be characterized with x,y coordinates on the 1931 CIE Chromaticity Diagram or with u′, v′ coordinates on the 1976 CIE Chromaticity Diagram.
In the 1931 Diagram, deviation from a point on the Diagram (i.e., “color point”) can be expressed either in terms of the x, y coordinates or, alternatively, in order to give an indication as to the extent of the perceived difference in color, in terms of MacAdam ellipses. For example, a locus of points defined as being ten MacAdam ellipses from a specified hue defined by a particular set of coordinates on the 1931 Diagram consists of hues that would each be perceived as differing from the specified hue to a common extent (and likewise for loci of points defined as being spaced from a particular hue by other quantities of MacAdam ellipses).
A typical human eye is able to differentiate between hues that are spaced from each other by more than seven MacAdam ellipses (but is not able to differentiate between hues that are spaced from each other by seven or fewer MacAdam ellipses).
Since similar distances on the 1976 Diagram represent similar perceived differences in color, deviation from a point on the 1976 Diagram can be expressed in terms of the coordinates, u′ and v′, e.g., distance from the point=(Δu′2+Δv′2)1/2. This formula gives a value, in the scale of the u′ v′ coordinates, corresponding to the distance between points. The hues defined by a locus of points that are each a common distance from a specified color point consist of hues that would each be perceived as differing from the specified hue to a common extent. Accordingly, the expression “a first color point that is within 0.01 delta u′, v′ of a second color point” means that the distance between the first color point and the second color point is not more than 0.01 in the scale of the u′ and v′ coordinates in a 1976 CIE Chromaticity Diagram, i.e., the square root of (the sum of the square of the difference in the respective u′ coordinates and the square of the difference in the respective v′ coordinates) is not more than 0.01, and analogous expressions have analogous meanings.
A series of points that is commonly represented on the CIE Diagrams is referred to as the blackbody locus. The chromaticity coordinates (i.e., color points) that lie along the blackbody locus obey Planck's equation: E(λ)=A λ−5/(e(B/T)−1), where E is the emission intensity, λ is the emission wavelength, T is the color temperature of the blackbody and A and B are constants. The 1976 CIE Diagram includes temperature listings along the blackbody locus. These temperature listings show the color path of a blackbody radiator that is caused to increase to such temperatures. As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally blueish. This occurs because the wavelength associated with the peak radiation of the blackbody radiator becomes progressively shorter with increased temperature, consistent with the Wien Displacement Law. Illuminants that produce light that is on or near the blackbody locus can thus be described in terms of their color temperature.
The emission spectrum of any particular light emitting diode is typically concentrated around a single wavelength (as dictated by the light emitting diode's composition and structure), which is desirable for some applications, but not desirable for others, (e.g., for providing general illumination, such an emission spectrum by itself would provide a very low CRI Ra).
In many situations (e.g., lighting devices used for general illuminations), the color of light output that is desired differs from the color of light that is output from a single solid state light emitter, and so in many of such situations, combinations of two or more types of solid state light emitters that emit light of different hues are employed. Where such combinations are used, there is often a desire for the light output from the lighting device to have a particular degree of uniformity, i.e., to reduce the variance of the color of light emitted by the lighting device at a particular minimum distance or distances. For example, there may be a desire for “pixelation”, the existence of visually perceptible differences in hues in the output light, to be reduced or eliminated at a particular distance (e.g., 18 inches) from a lighting device (e.g., by holding up a sheet of white paper and seeing whether different hues can be perceived), i.e., for adequate mixing of the light emitted by emitters that emit light of different hues to be achieved.
The most common type of general illumination is white light (or near white light), i.e., light that is close to the blackbody locus, e.g., within about 10 MacAdam ellipses of the blackbody locus on a 1931 CIE Chromaticity Diagram. Light with such proximity to the blackbody locus is referred to as “white” light in terms of its illumination, even though some light that is within 10 MacAdam ellipses of the blackbody locus is tinted to some degree, e.g., light from incandescent bulbs is called “white” even though it sometimes has a golden or reddish tint; also, if the light having a correlated color temperature of 1500 K or less is excluded, the very red light along the blackbody locus is excluded.
Light that is perceived as white can be made by blending two or more hues (or wavelengths).
“White” solid state light emitting lamps have been produced by providing devices that mix different colors of light, e.g., by using light emitting diodes that emit light of differing respective colors and/or by converting some or all of the light emitted from the light emitting diodes using luminescent material. For example, as is well known, some lamps (referred to as “RGB lamps”) use red, green and blue light emitting diodes, and other lamps use (1) one or more light emitting diodes that generate blue light and (2) luminescent material (e.g., one or more phosphor materials) that emits yellow light in response to excitation by light emitted by the light emitting diode, whereby the blue light and the yellow light, when mixed, produce light that is perceived as white light. While there is a need for more efficient white lighting, there is in general a need for more efficient lighting in all hues.
Accordingly, there is an ongoing need for light sources that are more efficient than existing options and that provide good CRI Ra over the lifetime of the light sources.